From: "Mike Collins" <Mike AT NO_SPAM DOT e-col DOT freeserve DOT co DOT uk>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: A bug in DJGPP? (length of bit fields)
Date: Sat, 1 Jan 2000 23:11:20 -0000
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To: djgpp AT delorie DOT com
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> >Yes, I know that a bit field can be defined as short, in which case, it
> >would occupy a short int, equal to 2 bytes, but in my structure, I
defined
> >the bit fields inside an unsigned int. Shouldn't that force the length of
> >the bit fields to be 4, and any other stuff in the structure should be
added
> >to that?
> >
>
>   But the bit fields did not use all the space so the compiler is free to
> add or subtract additional space as it see fit.
> Since in thoeory you are not using this extra space. If you are using
> the extra space you should define it.
>

Thanks, David, I think I had misunderstood the meaning of the definition of
a bit field. I don't have a copy of K&R, but I looked it up in the library.
I had previously learned that when I defined :

unsigned bit_field: 6;

.. it meant that 6 bits of an unsigned int would be used for my bit field.
The length used up in the structure would necessarily be sizeof)unsigned). I
now reallise that it is the 6 bit field which contains an unsigned number,
as it might have contained a signed one. I had not reallised that you could
define:

int bit_field_1:3,
     bit_field_2:6;

... and that both bit fields would contain signed numbers.

Well, you live and learn.

My thanks to yourself and Phrasant, who also spent time with me on e-mail
trying to sort this out, and my apologies to DJGPP, which doesn't have a bug
after all!

Mike.